1. Field of the Invention
The present invention relates generally to threaded connections used in securing two components together. More specifically, the present invention relates to a threaded pin and box connector for assembling tubular pipe bodies to each other.
2. Background Setting of the Invention
The pipe used in the construction of wells is usually in the form of a long tubular string assembled from a number of small pipe sections. The pipe can be used as a drill string, a casing or tubing string, a tendon for offshore rig anchoring or other applications requiring connection together of individual threaded components. The pipe sections are secured together at their ends by an externally threaded connector, or “pin” that is threadedly received within an internally threaded connector or “box”. Each pipe section has a pin on one pipe end and a box at the opposite pipe end. Some pipe has an internally threaded coupling secured to one end of a double pin pipe section to produce the box. The individual pipe sections are frequently referred to as a “pipe joint”. Tubing and casing pipe joints are usually 30 ft. in length. Casing pipe joints typically vary in length from 20 ft. to 40 ft. or longer.
The various pipe strings used in constructing a well are usually assembled on the floor of a drilling or workover rig. The pipe string is lengthened and lowered into the well as succeeding pipe joints are added to the string. During this assembly procedure, the pipe joint being added to the string is lowered, pin down, into an upwardly facing box projecting from the drilling rig floor. This procedure is commonly referred to as “stabbing” the pin into the box. After being stabbed, the added pipe joint is rotated to engage the threads of the pin and box, securing the joint to the string.
In disassembling a string, the previously described process is reversed. The pipe joint at the top of the string is rotated to disengage the pin and box threads. Once the threads have disengaged, the pin is lifted out of the upwardly facing box projecting from the rig floor. This procedure is sometimes referred to as “destabbing” the pin from the box. Once free of the box, the removed joint is moved to a storage location.
Thread damage can occur if the pin is not cleanly stabbed into the box of the connection. If there is misalignment between the pin and box as the joint is lowered into engagement with the box, the face and side of the pin can strike seals and threads in the box causing damage to both components of the connection. The use of a stabbing guide when a joint is being added to the string minimizes stabbing damage. The stabbing guide centralizes the pin over the box and prevents the pin from striking the box threads or seals as the pin is being stabbed into the box.
A problem with disassembling certain threaded pipe strings is that the pin and box threads can hang on each other when the pipe joints are being destabbed after having been unthreaded from each other. A hang-up of the pin and box threads during destabbing can seriously damage both components of the threaded connection and can also cause inadvertent release and dropping of the entire pipe string or injury to personnel.
The probability of a hang-up is increased in multi-step thread forms where the threads are machined on adjoining cylindrical surfaces having dissimilar diameters. A multi-step thread design is beneficial in that it permits a threaded pin to be stabbed deeply into a threaded box without relative rotation between the two pipe joints. Once the pin is stabbed, threaded engagement of multi-step pin and box threads is effected by a relatively few number of turns of the pin into the box. By contrast, full makeup of non-tapering single step connections requires that the pin and box threads be rotated relative to each other a number of revolutions that equals the number of connection thread revolutions.
Certain two-step drill pipe connections currently used in prior art designs employ a two-step wedge thread. A two-step thread design that employs a wedge thread is particularly susceptible to hang-up during stabbing and destabbing of the connection. The increased susceptibility results from the fact that the wedge thread tooth changes in width over its length so that a relatively narrow pin thread can come into registry with relatively broad spacing between adjacent box thread teeth during stabbing and destabbing. If the centerline of the pin is displaced from the centerline of the box during the stabbing and destabbing motion, the threads near the end of the pin will hang-up in the threads near the face of the box. When the threads hang-up during destabbing, it is necessary to slightly lower the joint, center the pin within the box and then lift the pin free of the box. If large forces are exerted on threads after they have hung up, serious damage may be done to the threads of the pin and box.
The special care required to prevent hanging up of the threads when a drill string is being laid down or placed back in the derrick is time consuming and can unacceptably extend the time required to pull the drill string from the well. The designers of the prior art wedge thread drill pipe address the problem by providing a centralizing destabbing guide that can be placed over the connection before the unscrewed pin is lifted out of the box. Proper placement of the destabbing guide around the connection requires additional personnel effort and extends the time required to remove the pipe string from the well. Moreover, the centralizing guide may not be effective in maintaining centralization of the pin and box under certain conditions such as occur, for example, when a strong side wind is present or when the drilling rig top drive or block is not properly centered over the drill string box.
Constructing the thread diameters of two-step connections with as large a difference in diameters as possible reduces the probability of thread hang-up during stabbing and destabbing. This design tactic permits greater centerline displacement to occur between the pin and box before the thread teeth on the side of the pin engage the thread teeth on the inside of the box. However, in order to achieve this reduction in hang-up without weakening the connection, the external dimensions of the connector must be increased and/or the internal passage through the connector must be decreased. Good string design dictates that the external and internal diameters of a threaded connector be maintained as closely as possible to the external and internal diameters, respectively, of the pipe bodies being secured together by the connector. Extending the outside diameter of a connector or reducing the internal clearance through the connector to minimize the incidence of thread hang-up on stabbing or destabbing compromises optimum connection design.